At least 50 people died and dozens more were injured at a Florida nightclub in the worst mass shooting in recent US history.

Following reports a lone gunman carried out the attack after pledging allegiance to Islamic State, former Republican presidential nominee Marco Rubio warned the incident represented the “new face of the war on terror”.

Gunman Omar Mateen opened fire on revellers at gay bar Pulse in Orlando in the early hours of yesterday morning, before being shot dead by officers.

President Barack Obama said Americans were united in grief, outrage and “resolve to defend our people” after “an act of terror and an act of hate”.

“We must spare no effort to determine, what, if any, inspiration or association this killer may have had with terrorist groups.”

It is thought Mateen, 29, who worked as a security officer, was operating alone. He was not thought to be connected with any particular radical group.

Authorities said the US citizen, of Afghan descent, was not on the country’s terrorism watch list but was being investigated for an unrelated crime.

NBC News reported that Mateen had called the emergency services before the attack and swore allegiance to the Islamic State militant group.

The group later said, via its affiliated Amaq news agency, that an IS “fighter” had carried out the attack.

Meanwhile local media quoted his father, Mir Saddique, as saying his son, who lived in Florida, had become angry after seeing gay men kissing on a family trip to Miami.

“We are in shock like the whole country. This had nothing to do with religion,” Mr Seddique added.

Orlando mayor Buddy Dyer described the incident as ­“something we never imagined and is unimaginable”.

He revealed that there was “an enormous amount of havoc” at the site of the killings which was left with “blood everywhere”.

Last night, hundreds of people gathered at hospitals desperate for news of their loved ones, many holding images of missing relatives and friends. There were huge queues outside wards as residents lined up to donate blood to help victims.

Carrying a device believed to be a bomb and armed with an assault rifle and a handgun, Mateen opened fire in the club and took hostages before becoming embroiled in a three-hour stand-off with police.

When officers stormed the building the gunman was said to have been hiding in a toilet with hostages.

Florida Senator Marco Rubio warned the incident “was a reminder the war on terror has evolved into something we’ve never had to confront before – individuals capable of conducting massacres unexpectedly in places you normally wouldn’t associate with the kinds of attacks you see”.

“One of the hardest parts of this war is the individual who carries out an attack by themselves in a soft target like this” he added,

Doctors last night said the death toll was likely to rise with 53 people being treated for injuries, many on the critical list.

A number of club goers described how they managed to escape the gunman by climbing out side doors.

Ricardo J Negron Almodovar wrote on social media: “I was there. Shooter opened fire. People on the dance floor and bar got down on the floor, some of us managed to go out through outdoor area and just run.”

Fellow clubber Christopher Hansen described the horrific scenes in the aftermath. He said: “There were just bodies everywhere. In the parking lot, they were tagging them – red, yellow – so they knew who to help first. Pants down, shirts cut off, they had to find the bullets. Just blood everywhere.”

Juan Rivera who was in the club at the time of the attack said the gunman arrived at about 1am, saying “The club was about to close, we heard shooting, about 30 shots and all of a sudden the lights went off and we had blood all over our shirts.”

Mr Rivera said the gunman did not speak and “was just firing as he was blocking one of two exits from the club”.

Fatriana Evans said she heard a “pop, pop, pop”, describing the hail of bullets as like “fireworks”.

Another witness said many people ripped off their clothes to use as bandages on victims.

Muhammad Musri, of the Islamic Society of Central Florida, told a press conference the community was “in mourning and heartbroken”.

He said: “No-one could have predicted this. It’s like lightening. It can strike anywhere.”

He added: “This shooter is not connected with other people, so citizens should continue to feel safe.”

Police special agent Danny Banks described the attack as an “act of terrorism” and said: “Whether that is domestic terrorist activity or an international one, that is something we will certainly get to the bottom of.”

Florida governor Rick Scott called the shooting “heart-wrenching” and said people who went into the building knowing there was an active shooter are heroic.

American media last night quoted the ex-wife of the gunman as saying he was violent and unstable.

Republican presidential hopeful Donald Trump said he was praying for victims’ families, and added: “Appreciate the congrats for being right on radical Islamic terrorism, I don’t want congrats, I want toughness & vigilance. We must be smart.”

French president Francois Hollande condemned the mass killing “with horror” and expressed the “full support of France and the French with America’s authorities and its people in this difficult time”.

David Cameron said on Twitter: “I’m horrified...My thoughts are with the victims and their families.”